I've found two sources to dispute your suggestion that Robert McCandlish was a son of William McCandlish and not the family immigrant himself.
The first is from the book SCOTS ON THE CHESAPEAKE (c. 1992, compiled by David Dobson).In the book is a single line notation on page 89 (and I can't find the copy I made of that page so this is from memory) that says Robert McCandlish emigrated from Wigtownshire in 1760, was a tutor, and landed in Essex Co, VA.[The next time I see that book I'll re-copy the citation page!]
The other source is from a small booklet I found in the Norfolk, VA public library: THE STORY OF TODDS BRIDGE--DUNKIRK - An Account of the Rise and Decline of an Old Mattaponi River Settlement, by James Mason Grove (c. 1983, Williamsburg, VA).It contains, amazingly, an entire chapter about Robert McCandlish"In Search of Robert McCandlish".
The first paragraph of that chapter reads as follows: "According to an 'excursus', reference to which was found in Swem's Historical Index, and to family papers in the possession of Robert J. McCandlish, Jr.,of Fairfax, VA., Robert McCandlish, first of his name in the Colony, a lowland Scotsman of Wigtonshire, came to Essex County, Virginia in 1760.He was educated as a schoolamster and very probably taught school in Essex for some few years.Later, he moved to King and Queen County, where circumstancial evidence suggests that he taught school, and primary couces indicate that he was an agent or factor for a firm of Scottish merchants.Here he married Elizabeth Coleman, born circa 1756, daughter of Thomas Coleman, sometime high sheriff, justice, and member of the Committee of Safety of King and Queen, and Mildred Richards his wife."
I would be interested to see what you've found on the McCandlish family.Robert was my g-g-g-g-grandfather.